I'm cutting a piece of 12-gauge Romex that has some extra slack in it, and I'm inserting a box with two receptacles (so I guess electricians call that a double duplex).
I was and found this (http://www.wikihow.com/Install-an-El...t-from-Scratch step #9):
Connect the ground wire. If a metal box, cut (2) eight inch lengths of bare copper wire, and combine them and the bare wire from the romex cable under a properly sized wirenut. The free end of one wire will connect to the green ground screw of the outlet, and the other free end is to be connected to the metal box with a green screw (expressly for that purpose) or other approved means (special clips, etc). If a plastic (or other non-conductive material) box is used, simply connect the bare copper wire directly under the green screw of the outletI'm using a metal box and a metal cover, with metal screws connecting the two.
Do I really have to wire a ground to the receptacle, and another ground to the metal box? Or would it be sufficient to wire the two grounds to the box and let the metal screws carry the ground to the receptacles?
My layman thinking is that the screws would be sufficient for the ground. But I may be overlooking a scenario in which they wouldn't, and I want to get it right.
But if conduit can serve as a ground, why can't my all metal box (box and cover that holds the receptacles) do the same thing?
The only answer I can think of is that if someone unscrews the cover and pulls it away from the box, they have broken the ground. But if they are pulling the cover off with the circuit live, the ground may not save them anyhow.
What should I do?